Fix lints: unnecessary_brace_in_string_interp

pkg/analysis_server/test/services/index/dart_index_contributor_test.dart

Index: pkg/analysis_server/test/services/index/dart_index_contributor_test.dart
diff --git a/pkg/analysis_server/test/services/index/dart_index_contributor_test.dart b/pkg/analysis_server/test/services/index/dart_index_contributor_test.dart
index 476c4dd81fbb1b9cbf2be9a61e9a6d8c9d380c60..5ebcc476333044a2c7d08459741910ac7dad54c3 100644
--- a/pkg/analysis_server/test/services/index/dart_index_contributor_test.dart
+++ b/pkg/analysis_server/test/services/index/dart_index_contributor_test.dart
@@ -1665,7 +1665,7 @@ main(A a, p) {
     for (RecordedRelation recordedRelation in recordedRelations) {
       if (_equalsRecordedRelation(
           recordedRelation, expectedIndexable, relationship, location)) {
-        fail('not expected: ${recordedRelation} in\n' +
+        fail('not expected: $recordedRelation in\n' +
             recordedRelations.join('\n'));
       }
     }